streamingest: support reversing replication direction

[dt]

After promoting a standby that was replicating from some primary to be
its own active cluster, turning it into the new primary, it is often
desirable to reverse the replication direction, so that changes made to
this now-primary cluster are replicated back to the former primary,
now operating as a standby.

Turning a formerly active, primary cluster into a replicating standby
cluster is particularly common during "failback" flows, where the once
primary cluster is returned to primary status after the standby had
temporarily been made the active cluster.

Re-promoting the primary in such cases requires it have a virtual cluster
that is fully caught up with the promoted standby cluster that is serving
traffic, then performing cut-over from that standby back to the primary.

This could be performed by creating a completely new virtual cluster
in the primary cluster from a replication stream of the temporarily active
standby; just like the creation of a normal secondary replicating cluster
this would start by backfilling all data from the source -- the promoted
standby -- and then continuously applying changes as they are streamed
to it.

However, in cases where this is being done on a cluster that previously
was the primary cluster, the cluster may still have a nearly up to date
copy of the virtual cluster, with only those writes that have been applied
by the promoted standby after cutover missing from it. In such cases,
backfilling a completely new virtual cluster from the promoted standby
involves copying far more data than needed; most of that data is already
on the primary.

Instead, the new syntax ALTER VIRTUAL CLUSTER a START REPLICATION FROM a ON x
can be used to indicate the virtual cluster 'a' should be rewound back to
the time at which virtual cluster 'a' on physical cluster 'x' -- the promoted
standby -- diverged from it. This will check with cluster x to confirm that
its virtual cluster a was indeed replicated from the cluster running the
command, and then communicate the time after which they diverged, once
cluster x was made active and started accepting new writes. The cluster
rewinds virtual cluster x back to that timestamp, then starts replicating
from cluster x at that timestamp.

Release note (enterprise change): A virtual cluster which was previously being
used as the source for physical cluster replication into a standby in another
cluster which has since been activated can now be reconfigured to become a
standby of that now-promoted cluster, reversing the direction of the replication
stream, and does so by reusing the existing data as much as possible.

Epic: CRDB-34233.

[cockroach-teamcity]

This change is 

[dt]

First couple commits are #117636 so I'd review that PR first.

[dt]

rebased on #117636 and RFAL

[dt]

TFTR!

bors r=msbutler

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)